The present invention relates to a marine riser and to methods of using such a riser. The marine riser is useful for a variety of possible applications in the offshore oil and gas industry, but is particularly intended for use in the drilling, servicing (xe2x80x9cwell interventionxe2x80x9d) and abandonment of subsea well installations.
There is a need for a variety of maintenance and service operations to be carried out on subsea wellheads, following completion of the well and throughout the operational lifetime of the well. Many of such operations require a conduit (xe2x80x9criserxe2x80x9d) to connect the wellhead to the surface of the water, allowing coiled tubing or the like to be introduced into the bore of the well, through the riser. Conventionally, such operations have usually been performed using a riser formed from rigid steel drill pipe deployed from a conventional drilling rig (typically a mobile semi-submersible type rig). This has numerous disadvantages. Such rigs are expensive, slow in transit between tasks at different locations and cumbersome in use.
It would be desirable to carry out such operations using a conventional, dynamically-positioned drilling vessel, equipped with a standard oilfield derrick. Difficulties arise when using such a vessel with a conventional rigid riser, primarily because a vessel of this type is substantially less stable than a semi-submersible rig. In order to use such a vessel for the deployment of marine risers it is necessary to control bending moments arising from environmental loads on the riser and from roll, pitch, sway and yaw of the vessel.
This problem has been addressed in the past in a variety of ways, including:
(a) Rigid risers manufactured from high performance materials and/or with complex geometries which can absorb the bending forces. This approach is expensive in terms of materials and manufacturing costs.
(b) Application of extremely high tensions to the riser. This creates a whole range of other problems.
(c) Forming the riser wholly from flexible pipe. Such pipe is expensive, and the length of the riser must match the water depth quite closely, so that a range of different lengths will be required for different operations. A storage carousel for the flexible pipe is also required on the vessel, where deck space is limited.
(d) The use of xe2x80x9cflex-jointsxe2x80x9d, such as those marketed by Oil States Industries of Arlington, Tex., USA. A joint of this type comprises a short articulated conduit with a flexible coupling connecting two rigid conduit sections, one of which includes a massive collar enclosing an elastomeric bearing. Devices of this type are bulky, massive and extremely expensive, and accommodate only a limited range of riser deflections (typically +/xe2x88x9210xc2x0).
It is an object the invention to provide a marine riser which can be deployed from a conventional oilfield rig on a conventional dynamically-positioned drilling vessel and which obviates or mitigates the various problems outlined above. The riser may also be useful in other fields of application within the offshore engineering industry.
In accordance with a first aspect of the invention, there is provided a marine riser in which at least part of the length of the riser is formed from at least one length of rigid tubular pipe and at least part is formed from at least one length of flexible pipe.
In its preferred embodiment, the riser comprises a central rigid section and uppermost and lowermost flexible sections.
The at least one rigid section preferably comprises a plurality of rigid pipe joints assembled together to make up the length required and the at least one flexible section is pre-fabricated to a predetermined length.
The at least one flexible section may be provided with bend restricting devices adapted to resist bending and/or bend limiting devices adapted to limit the minimum radius to which the flexible pipe may be bent.
The various flexible and rigid sections may be connected to one another by any suitable means, including flange, hub and screw-threaded connectors, The ends of the riser are adapted for connection to subsea installations and to apparatus on board the vessel, respectively, as required for a particular operation. The lowermost end may have a package of apparatus connected thereto for connection to the subsea installation.
In accordance with a second aspect of the invention there is provided a method of deploying a marine riser between a vessel and a subsea installation, comprising lowering a riser from the vessel to the subsea installation and connecting the lower end of the riser to the subsea installation, wherein the riser includes at least one length of rigid tubular pipe and at least one length of flexible pipe.
Preferably, said at least one length of rigid tubular pipe comprises a plurality of pipe joints which are connected together as the riser is lowered from the vessel.
Preferably also, the method comprises lowering a first length of flexible pipe, connecting a first rigid pipe joint to an upper end of said flexible pipe, lowering said rigid pipe joint, connecting additional rigid pipe joints to the upper end of the preceding pipe joint and lowering said additional pipe joints, as required, connecting a second length of flexible pipe to the upper end of the last rigid pipe joint and lowering said second length of flexible pipe.
Preferably also, the vessel is a dynamically positioned vessel and the pipe is lowered from a derrick located on the vessel, via a moon-pool.